Some pathogens are common and dormant and do not harm, until they are activated and become virulent due to use of certain pharmaceuticals. This has been proven in the HPV vaccine -- many people have become sick with Bartonella or Lyme's disease after vaccination. The pathogens were already laying dormant in them, perhaps for years, with no ill effect. But the HPV vaccine activated them in some susceptible people. Bartonella is present in 50% of household cats in the USA, and usually causes no harm.
Here's an interesting article explaining the above:
http://sanevax.org/gardasil-and-bartonella-a-dirty-little-secret/